The French police have busted a gang that allegedly smuggled Vietnamese into France to work clandestinely on cannabis farms, French news website Dernières Nouvelles d'Alsace said.

A Vietnamese woman and her two sons are thought to be the leaders the ring that was busted in Alsace in eastern France.

Thirteen people were arrested, but four have been released.

An AFP report said that six of the suspects are from the same family, adding that the illegal immigrants were also sent to work on drug farms in the UK.

The police said the gang brought in the Vietnamese through Paris and found them accommodation before sending them to Alsace to work on three cannabis farms.

Julien Gentile of the Central Office for Repression of Illegal Immigration and Employment of Untitled Foreigners was quoted as saying by the website that the Vietnamese worked to pay the fees of 20,000-30,000 euros (US$25,900-38,800).

A farm in Ingwiller town in Alsace, which has around 1,700 cannabis plants, yields three or four crops a year for the annual profit of 350,000-400,000 euros ($453,000-517,700), he said.

The investigation was triggered after the arrest in September of six Vietnamese traveling on a Lisbon-Paris train with forged travel documents, AFP reported.

It said the migrants told police they had been brought to Europe by an agency in Hanoi which was ostensibly recruiting workers to pick fruit in Portugal.